FIG. 1 shows a conventional antenna apparatus. As shown in FIG. 1, the sub-reflector 2 is situated on a principal axis of the main reflector 1 opposite to the same, and the primary horn 3 is situated between the sub-reflector 2 and the main reflector 1.
In case that an electric wave is transmitted by the antenna apparatus shown in FIG. 1, a transmitting power is supplied to the primary horn 3 from a transmitter (not shown). The electric wave radiated from the primary horn 3 illuminates the sub-reflector 2, and is reflected thereby to the main reflector 1. The electric wave reflected by the sub-reflector 2 is again reflected by the main reflector 1, converted into a parallel beam, and transmitted to a receiving antenna apparatus passing along a path 13A to 13B for the transmitting wave.
However, according to the aforementioned conventional antenna apparatus, both the desired and undesired waves are reflected by the sub-reflector 2 and the main reflector 1, and radiated to the outside passing along the path 13A to 13B for the transmitting wave. Accordingly, the undesired wave is radiated to the outside of the antenna apparatus also.